Overstay in Prague by one day

This summer, my friend from Thailand will come to see me in Prague, Czech Repulic. He applied for a Schengen tourist visa type C and received it, valid for 30 days with 15 days of the stay.

Due to the miscalculations, he booked the following flights:
Arrival in Prague 28th July, 13:30 (1:30 pm)
Departure from Prague 12th August, 12:30 (12:30 pm, half past noon).

Overall it makes 16 days. He tried to change the flight back on 11th August, but is having problems.

The travel agency fees are too high to change the flight as well as the cost of the confirmation of the change for the international calls. Moreover, it isn't replying to his messages, so it is unlikely that he'll change the flight.

What are consequences of overstaying in Prague by 12 hours (technically one day) and what is the likely scenario? What can we do except canceling the flight? Is there a way to extend the duration of stay in Prague?

Best Answer

A ban is probably legally possible but very unlikely. I assume a fine is also possible but not certain. With a bit of luck, they might also get away with a slap on the wrist. It could however become more difficult to secure another visa in the future.

But the consequences of the overstay is not the only thing you have to worry about: Border guards are perfectly entitled to examine your plans upon entering. If they notice the discrepancy, they can also revoke the visa and send your friends back to Thailand on the grounds that they don't have proper documentation for their intended stay. That would be much worse on every level.